1. Technical Field of the Invention
The present invention relates to an automatic exposure control apparatus that automatically controls exposure by utilizing an output signal of a solid-state imaging device.
2. Description of the Related Art
Recently, there has been increasing use of electronic image-capture apparatuses that use an imager, which employs a solid-state imaging device to capture subject images, and that record the captured images in a storage medium of semiconductor memory or magnetic tape.
To automatically set an image-capture optical system in an image-capture apparatus such as that described above under appropriate exposure conditions, an automatic exposure control apparatus is employed.
As a type of automatic exposure control apparatus, there is an apparatus that makes use of an output from a charge-coupled device (hereinafter referred to as a “CCD”) to perform automatic exposure control (a so-called “imager AE”); more particularly, there is an apparatus that integrates an automatic exposure sensing area of an image output from a CCD and refers to the integration result to determine exposure conditions (such as shutter speed and diaphragm aperture adjustment).
FIG. 5 is a timing chart illustrating an example of an automatic exposure control operation of a conventional automatic exposure control apparatus. In this figure, each of the horizontal axes represents time and is separated into two sections because of limited space, the right end of the axis in the upper section continuing to the left end of the axis that has the same alphabetical letter (i) (i=a, b, . . . , f) in the respective lower section. Other figures such as FIG. 2 are displayed in the same way.
In FIGS. 5,(a)–(f) respectively represent a mecha-shutter, CCD readout pulse ON/OFF (ON permits a readout-pulse output; OFF inhibits a readout-pulse output), exposure time, CCD output, AE area integration, and AE operation.
The mecha-shutter is kept in an OPEN state until it receives a shutter trigger, but it changes to a CLOSE state upon receipt of a shutter trigger.
Regarding the CCD readout pulse, it is transferred from a photosensitive area (photoelectric conversion area) of the CCD, and it is always kept in an ON state.
After an exposure time TI, a signal charge of the CCD photoelectric conversion area is then transferred to a transfer area as a CCD readout pulse, the transferred signal charge has vertical and horizontal transfer pulses applied to it, and then it is output from the CCD.
The CCD output signal which corresponds to an AE sensing area is subjected to an integrating operation. This is indicated as the integration value EI+SI as an AE area integral. Here, EI represents an integration value of net signal charge accumulated in the CCD photoelectric conversion area (the AE sensing area of) at the exposure period TI; SI represents an integration value of a so-called smear occurring in a vertical transfer area because of oblique incidence transmission of a bright subject image and the like.
An operation area for performing an automatic exposure operation performs an AE operation to compare the integration value EI+SI to a target value. If EI+SI>target value, the operation area then determines the next exposure time TI+1 so as to be TI>TI+1; while, if EI+SI<target value, it then determines the next exposure time TI+1 so as to be TI<TI+1.
In this way, the next exposure time is consecutively determined, and if a release operation is performed after an exposure time Tn and a shutter trigger are received, the exposure time required for actual image-recording is then determined from the integration value En+Sn; and after the exposure time is determined, it is further transferred as a CCD readout pulse to a transfer area, the mecha-shutter is shut off, and then a CCD output is produced through application of the vertical and horizontal transfer signals.
In this state, the mecha-shutter is kept shut; therefore, the smear integration value is zero (0). This is different from the condition in which the AE operation is performed by determining exposure time by keeping the mecha-shutter open.
Particularly, before performing exposure, the next exposure time has previously been determined when there has always been a smear component present existed; however, at the time of actual exposure, the exposure amount is reduced by the amount corresponding to the smear component when it is not present. This indicates that the exposure amount at an actual exposure time is set as En/(En+Sn), which is reduced to correspond to the smear component ratio.
For this reason, defects have occurred in that the larger the smear effect, the darker the captured image has been.
With the conventional example, in a bright scene in which the smear occurs and at the imager AE, the required exposure time has been determined on the basis of CCD output in consideration of smear inclusion, so that when images are captured by use of a camera allowing use of a mecha-shutter or diaphragm thereof to reduce the aperture, defective captured images being dark in proportion to the smear amount are produced.